The invention relates to a magnetic resonance imaging apparatus which is provided with a gradient coil system arranged in a vacuum isolated space to generate a magnetic gradient field in an imaging volume of the apparatus, and with an RF coil which at least partly encloses the imaging volume to generate an RF magnetic alternating field in the imaging volume.
An apparatus of this kind is known from U.S. Pat. No. 5,489,848.
A magnetic resonance imaging apparatus for medical purposes, also referred to as an MRI apparatus, is arranged to form images of cross-sections or so-called slices of a body. To this end, in such an apparatus a strong, steady, uniform magnetic field is generated in a volume intended for imaging, that is, the so-called imaging volume. A magnetic gradient field is superposed on the uniform field in order to indicate the location of the slice to be imaged. The atoms in the tissue present in the imaging volume are then excited by means of an RF field. The spin resonance signal formed upon relaxation of the excited atoms is then used to reconstruct an image of the slice indicated by means of the gradient field. The steady, uniform field, also referred to as the main field, is generated by means of a coil system (superconducting or not). This coil system and the associated envelope together may be shaped as a short tube in which the imaging volume is situated. The diameter of this tube is determined by the dimensions of the patients to be examined and hence, has a given minimum value, for example, of the order of magnitude of 90 cm.
The gradient coil system for generating a magnetic gradient field in the imaging volume is arranged within the tube so as to enclose the imaging volume. The gradient coil system includes sets of gradient coils for generating an associated gradient field, i.e., for example, one set for each of the three co-ordinate directions, each system of gradient coils being referred to as a primary gradient coil. Three axial fields with gradients in the three coordinate directions x, y and the axial direction z are thus produced. During the imaging process current pulses are applied to the gradient coils so that magnetic stray fields occur inevitably outside the imaging volume. These stray fields are liable to induce eddy currents in the conductive parts of the apparatus which are situated in the vicinity of the gradient coils, notably the metal parts provided so as to generate the main field, for example the tubular part of the envelope for the coil system, any heat shields situated within the envelope (in the case of a superconducting coil system), or the coils for the main field itself. The magnetic fields produced by the eddy currents cause distortions in the image to be formed. They also cause heat dissipation in the parts conducting the eddy currents. This is a drawback notably in the case of a superconducting coil system, because the liquid helium which serves as the cooling medium boils down faster. Finally, they also cause a disturbing noise since the parts conducting the eddy currents are situated in a magnetic field and hence, are subject to Lorentz forces causing deformation of the parts. In order to counteract the adverse effects of such stray fields, shielding coils are used so as to shield or compensate the magnetic fields generated outside the imaging volume by the primary coils. The current directions in the primary coils and the shielding coils oppose one another and hence, these coils are also subject to oppositely directed Lorentz forces. In order to minimize the vibrations due to such forces, these two coils are mechanically interconnected so as to form one rigid unit. However, the forces of the coils do not fully compensate one another. Consequently, a shielded coil also starts to vibrate and hence produces noise, so that there will still be a noise pollution which is experienced as being excessive. Consequently, the gradient coil system having primary coils and shielding coils is accommodated in a vacuum isolated space in the MRI apparatus disclosed in the cited U.S. Pat. No. 5,489,848.
According to the cited United States patent above, the RF coil for generating the magnetic alternating field is situated within the coils for generating the main field and the gradient coil system and encloses the imaging volume. The RF coil, therefore, is situated in an atmospheric space.
It is a drawback of the MRI apparatus described in the cited U.S. Pat. No. 5,489,848, however, that a substantial part of the power of the RF alternating field, obtained by means of the RF coil, is dissipated in the gradient coil system.
Therefore, it is an object of the present invention to limit this power loss and to provide an MRI apparatus in which the RF power is efficiently used. It is another object of the present invention to implement the steps for minimizing the power loss according to the invention in a structurally advantageous manner.
The objects of the present invention are achieved in that the MRI apparatus of the kind set forth according to the invention is characterized in that an RF shield is arranged in the vacuum isolated space between the gradient coil system and the RF coil. As a result of the presence of an RF shield, the power of the RF alternating field is concentrated predominantly in the imaging volume.
The mounting of an RF shield is known from U.S. Pat. No. 5,179,338. However, therein the shield is mounted on the outer wall of the vacuum isolated space bounded by the primary gradient coils and the shielding coils. Therefore, these coils are not suspended resiliently so that, despite the vacuum created in the space within the gradient coil, the noise reduction achieved is not optimum. Furthermore, because a minimum distance must be maintained between the RF coil and the RF shield, the mounting of an RF shield outside the vacuum isolated space means that for the same imaging volume to be maintained the dimensions of the MRI apparatus will become slightly larger.
In a first embodiment the RF shield is rigidly mounted on the inner wall of the vacuum isolated space so that the RF shield can be readily correctly positioned relative to the RF coil. The RF coil in a second embodiment is rigidly mounted on the inner side of the gradient coil system, that is, on the side thereof which faces the wall of the vacuum isolated space. From a structural point of view this offers the advantage that the gradient coil and the RF shield can be arranged in the relevant space as one unit and that the minimum distance between the RF coil and the RF shield can be realized in a manner that benefits the dimensions of the MRI apparatus However, this unit should be accurately centered in the relevant space.
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiment described hereinafter and the accompanying drawing.